A micro-electromechanical system (MEMS) is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by using microfabrication processes mostly derived from integrated circuit fabrication processes. The developments in the field of MEMS process engineering enabled batch production of electrostatically tiltable MEMS micromirrors and micromirror arrays that can be used in visual displays, optical attenuators and switches, and other devices. There are at least three general micromachining techniques used to manufacture MEMS micromirror devices.
One such technique is based on so called bulk micromachining, in which the whole thickness of a silicon wafer is used for building micro-mechanical structures. Silicon is machined using various etching processes. Anodic bonding of glass plates or additional silicon wafers is used for adding features in the third dimension and for hermetic encapsulation. Three-dimensional mechanical micro-structures can be created using bulk micromachining. Detrimentally, the bulk micromachining technique is very complex and requires many process steps.
Another technique is based on so called surface micromachining, in which layers are deposited on the surface of a substrate as the structural materials to be patterned, instead of a three-dimensional processing of the substrate itself, which significantly simplifies the manufacturing processes involved. The original surface micromachining concept was based on thin polycrystalline silicon layers patterned as movable mechanical structures and released by sacrificial etching of an underlying oxide layer. This MEMS paradigm has enabled the manufacturing of low cost MEMS devices.
New etching technology of deep reactive ion etching (RIE) has made it possible to combine performance and versatility of bulk micromachining with in-plane operation of surface micromachining. This combination formed a basis of a third micromachining technique called high aspect ratio (HAR) micromachining. While it is common in surface micromachining to have structural layer thickness in the range of 2 microns, in HAR micromachining, the achievable thickness of MEMS devices is from 10 to 100 microns. The materials commonly used in HAR micromachining are thick polycrystalline silicon, known as epi-poly, and bonded silicon-on-insulator (SOI) wafers. Due to its versatility and efficiency, this combined technology is quickly becoming the technology of choice for manufacturing MEMS tiltable micromirror devices.
MEMS tiltable micromirror devices are often used in optical switch applications. When a MEMS device is actuated, a micromirror supported by the device is tilted about a working axis, which makes an optical beam falling thereupon to steer from one output optical port to another, thereby realizing the switching function. By having a plurality of output ports disposed along a single line, a multiport optical switch can be constructed. In a multiport optical switch, however, it is advantageous to have the micromirror also tiltable about a secondary axis perpendicular to the working axis, so that the micromirror can be tilted about the secondary axis during the switching process, to deflect the optical beam laterally and avoid transient optical signals from briefly appearing in output ports that are on the way of the optical beam being steered towards a desired optical port. Therefore, having a MEMS micromirror device tiltable about a pair of mutually orthogonal axes is highly desirable and advantageous from the standpoint of an optical switch application.
MEMS devices can be actuated using a variety of actuators. One frequently used actuator for a MEMS device is an electrostatic actuator having a static electrode called a “stator”, and a movable, for example rotatable or tiltable, electrode called a “rotor”. An electrostatic attraction force between the stator and the rotor, applied against a returning force of a spring and, or a hinge element on which the rotor is suspended, tilts or rotates the rotor supporting a micromirror, whereby the micromirror is tilted in a controllable, predictable way. A special care is taken not to exceed the elastic limit of the spring and, or the hinge element. When the elastic limit is not exceeded, millions or even billions of tilting cycles are achievable over a lifetime of a single MEMS device.
Perhaps the simplest electrostatic actuator is a pair of planar plates, one being the stator and the other being the rotor. As the plates attract, the rotor plate tilts and is brought closer to the stator. By making the rotor plate tiltable about two orthogonal axes, e.g. X and Y axes, and by providing two stator plates, one for each axis of rotation, a micromirror attached to the rotor plate can be made electrostatically tiltable in two orthogonal axes of rotation. For example, U.S. Pat. No. 6,934,439 in the name of Mala et al., assigned to JDS Uniphase Corporation and incorporated herein by reference, teaches a linear array of tightly-spaced “piano” MEMS micromirror devices for use in a wavelength-selective optical switch application. Each micromirror of the MEMS micromirror array of Mala et al. is tiltable about two perpendicular axes X and Y, by the use of two stator plates, one for each axis of tilt, and by the use of two pairs of torsional hinges connected to an “internal” gimbal ring structure at the center of the micromirror. The flexible torsional hinges of Mala et al. provide for a pivotal mounting of the micromirrors, wherein each micromirror is independently tiltable.
Referring to FIG. 1, a top view of a prior-art tiltable MEMS device 100 of Mala et al. is shown having a platform 102 for supporting a micromirror, not shown, an anchor post 104 for supporting the platform 102, a Y-hinge 106 rotatable about a Y axis, a gimbal ring 108, an X-hinge 110 rotatable about an X axis, two Y-electrodes 112 for tilting the platform 102 about the Y-axis, and an X-electrode 114 for tilting the platform 102 about the X-axis. The electrodes 112 and 114, as well as the anchor post 104, are disposed on a substrate 116. The hinges 106 and 110, although shown by straight lines for simplicity, are serpentine spring hinges the platform 102 is suspended upon. The platform 102 is suspended over the substrate 116. In operation, a voltage is applied between the platform 102 and one of the electrodes 112 to tilt the platform 102 about the Y axis, and a voltage is applied between the platform 102 and the electrode 114 to tilt the platform 102 about the X axis. Tilting the platform 102 about the two orthogonal axes X and Y allows for a two-dimensional steering of an optical beam reflected from a mirror coating, not shown, of the platform 102.
Limitations of the MEMS device 100 of the prior art and, correspondingly, many advantages offered by a MEMS device of the present invention, are better understood upon considering a typical task of steering of an optical beam by a MEMS micromirror for a wavelength selective optical switch application. Turning now to FIG. 2, an orthographic projection view of a MEMS micromirror 200 is presented, consisting of a plan View A and orthogonal side Views B and C. The micromirror 200 is tiltable about a Y axis and an X axis. An incoming optical beam 202 has an elliptical cross-section 204 seen in View A. The elliptical cross-section 204 of the beam 202 is preferable over a circular cross-section because, for a typical application of a tiltable MEMS micromirror device in a wavelength selective optical switch, many optical beams at different wavelength are positioned so as to have their cross-sections disposed along a common axis, in this case, the Y axis. Correspondingly, decreasing the cross-section of the optical beam 200 in a Y-direction is advantageous, since it allows one to accommodate more individual beams 200 and more mirrors 200 along the Y axis, thereby increasing the wavelength resolution of the wavelength selective switch device. However, decreasing the beam size in Y-direction increases the beam divergence in that direction. For example, by comparing projections of a reflected beam 206 in the Views B and C of FIG. 2, one can see that the beam 206 diverges more in the projection of View C than it does in the projection of View B. Increased divergence requires one to increase the tilt angle for a switching application since the beam must be steered by an angle exceeding the beam divergence angle. Thus, a minimum tilt angle θx for switching the optical beam by tilting about the X axis is larger than a minimum tilt angle θy for switching the optical beam by tilting about the Y axis.
A requirement for a comparatively large tilt angle about the X axis has important implications for a tiltable MEMS micromirror device. Referring back to FIG. 1, an ellipse 101 denotes the elliptical cross-section of an impinging optical beam. To steer said optical beam about the X axis, a voltage is applied to the electrode 114. Due to the electrode 114 being located closer to the X axis than the electrode 112 is to the Y axis, the created X-torque is smaller than the Y-torque created by applying a voltage to any one of the electrodes 112. To ensure a larger tilt angle as has been explained above, at a smaller torque, the X-hinges 110 are typically made much more “weak”, or flexible, than the Y-hinges 106.
The flexible X-hinges 110 of the MEMS device 100 of the prior art are the weakest structures of the entire MEMS structure shown in FIG. 1. The flexibility of the X-hinges 110, although required for proper functioning of the device 100, leads to serious drawbacks inherent to the device 100. First, pistoning effects are significant due to the weaker X-hinges 110. When a voltage is applied between the platform 102 and one of the electrodes 112, and, or between the platform 102 and the electrode 114, the platform 102 shifts towards the electrodes 112 and 114, which changes the gap between the platform 102 and the electrodes 112 and 114, resulting in a change of sensitivity of the angle of tilt about the X and the Y axis to the voltage applied. This change of sensitivity leads to cross-coupling between the X and the Y tilts. Herein, the term “cross-coupling” is understood as mutual influence of X and Y actuation, that is, the actuation of tilt of the platform 102 about the X and the Y axes. Second, the X-hinges, being the weakest mechanical link in the entire MEMS device 100, lower the overall device reliability by making the device 100 more susceptible to shock and vibration. Third, manufacturing process related misalignments between the electrodes 112 and the X axis defined by the X-hinges 110 cause tilting the platform 102 about the X axis, or so called “roll”, upon application of a voltage to one of the electrodes 112 to tilt the platform 102 about the Y axis. The weaker hinges 110 make this “rolling” effect more pronounced.
Yet another drawback of the MEMS device 100 of FIG. 1 is that, upon tilting the platform 102 about the X axis, the gap between the electrodes 112 and the platform 102 changes, not only due to pistoning, but also due to tilting of the platform 102 itself about the X axis. This results in a further increase of cross-coupling of the X and Y tilts of the platform 102 and changing of the actuator sensitivities upon applying a voltage to the electrode 114.
In addition to complicating calibration and control, the pistoning effect lowers reliability of optical switch devices based on MEMS micromirror devices, since micromirror perturbations caused by vibration and shock can impact the transfer characteristic of the MEMS devices, leading to degradation of optical characteristics of the optical switch devices, such as insertion loss and isolation.
It is therefore a goal of the present invention to provide a tiltable MEMS micromirror device, in which the cross-coupling and pistoning effects are reduced by an order of magnitude, as compared to prior-art devices.
The tiltable MEMS micromirror device of the present invention meets this goal. Advantageously, the area of the rotor and the stator electrodes of the MEMS device of the present invention may be further increased by at least 50%, without increasing micromirror size or spacing in a micromirror array. This results in a further improvement of reliability, since the increased actuator area results in an increase of the electrostatic force; therefore, stronger hinges may be used to support the micromirror, and stronger micromirror hinges enhance the overall reliability of the tiltable MEMS micromirror device.